1. Field of the Invention
Methods for making the cellulose component in lignocellulosic materials more accessible to micro-organisms, enzymes and the like are of great interest in many practical applications. These include, for example, using the lignocellulose as animal feed or treating it with enzymes so as to produce sugar. Due to the material's complex chemical structure, however, microorganisms and enzymes can not effectively attack the cellulose without pretreatment. The cellulose is described as `inaccessible` to the enzyme or bacteria. The effect is illustrated by the inability of cattle to digest wood.
It is the primary goal of this invention to reduce the problem of inaccessibility of cellulose within hardwoods and agricultural residues such as bagasse and straw so as to render the cellulose in such materials more accessible to attack by rumen bacteria, micro-organisms, enzymes and the like. This is done using a highly critical set of operating parameters for steam cooking the material. These conditions optimize the accessibility of the cellulose. This accessibility is measured by either the in vitro cellulose digestibility to rumen bacteria or by the yield of sugar when the lignocellulose is treated by cellulase enzymes.
Lignocellulosic materials have three major components: cellulose, hemicellulose and lignin. Cellulose is a linear polysaccharide built up to gluco-glycosidic bonds. It has a relatively well-ordered structure, is pseudo-crystalline, and has a high enough chain length to be insoluble in water or dilute acids and alkali at room temperature. Cellulose is the major structural component of the cell wall and can be isolated as fibre.
Hemicelluloses are non-cellulosic polysaccharides. They are built up mainly of sugars other than glucose, are generally poorly ordered and non-crystalline, and have a much lower chain length than cellulose. The hemicelluloses occur in intimate association with the cellulose in certain layers of the cell wall, as well as in close association with the lignin between cells.
Lignin is an aromatic polymer, phenolic in nature, and built up from phenylpropane units. It has no systematic structure. Lignin occurs mainly as an encrusting agent between the fibres and on the outer layers of the cell wall.
The cellulose in lignocellulosic material is poorly accessible to micro-organisms, to enzymes and the like. That is, the micro-organisms can not easily react with the cellulose. This is due to its close association with lignin and hemicellulose in the cell structure, and to its highly cross-linked and crystalline molecular structure. To improve the accessibility, one must rupture the cell and break the close association with lignin.
2. Brief Description of the Prior Art
It has been known for some time that steam cooking changes the properties of lignocellulosic materials. The original and most broadly reported work on steam cooking with respect to such materials as hardwoods was carried out by Mason and is reflected in his various U.S. Pat. Nos. 1,824,221; 2,645,633; 2,494,545; 2,379,899; 2,379,890; 2,759,856. Mason's processes generally involved an initial slow cooking at low temperatures to glassify the lignin. This was followed by a very rapid pressure rise and quick release. The pressurized material blown from his reactor through a die caused defibration of the wood and resulted in the "fluffy", fibrous material commonly used in the manufacture of "Masonite" boards.
While Mason's work was directed towards the preservation of fibre for board production, some of the more recent research in steam cooking has centered on breaking down the fibre structure so as to increase the material's rumen digestibility. This work reflects a desire to increase the cellulose accessibility and therefore shares the goal of the present invention. Contributions have been made by Algeo, Bender, Delong, and Jelks and will be outlined below. First, however, a general note about steam cooking is in order.
In any steam cooking procedure, there are certain well known facts. The pressure of saturated steam in a reactor vessel, for example, has a well-defined relationship with temperature. It is also well understood that an inverse relationship exists between cooking time and temperature. Thus, when a pressure range is stated in conjunction with a range of cooking times, the shorter times are associated with the higher pressures, and the longer times with the lower pressures.
Algeo (U.S. Pat. No. 3,667,961) describes work upon ligno-cellulosic materials such as straw and almond hulls to produce feeds with cooking carried out at relatively high pressures. Algeo used steam cooking and explosive release with equipment physically similar to Mason's, adjusting cooking times so as to cause a greater breakdown of the lignocellulosic bonds. The material produced had a fine "sponge-like" texture.
Algeo also tested a variety of non-lignocellulosic materials (almond shells, coffee grounds) and found pressures and cooking time ranges to be "very critical for each commodity" (Col. 11, line 56). He noted that from a digestibility perspective, catalyzing the hydrolysis reaction drastically over-processes the material and can cause undesireable sugar production. That is, he found that converting the cellulose to sugar was unnecessary and, in fact, harmful when the goal was to produce cattle feed. His process was therefore carried out without the addition of catalysts. Table 1 lists Algeo's obviously preferred range for straw, a lignocellulose equivalent in structure to the hardwood and bagasse materials taught as preferred herein, based upon a understanding of his Table F, col. 11 as requiring the shorter times for the higher pressures.
TABLE 1 ______________________________________ Pressure Time ______________________________________ 400 psig 90 sec. 500 psig 60 sec. ______________________________________
Jelks (U.S. Pat. No. 3,933,286) proposed a two-stage process for the production of animal feed which first involved oxidation in the presence of a metal catalyst, then hydrolysis with an acid catalyst. Both reactions were at low pressures with moderate cooking times. He found the oxidation served to break a portion of the "lignin-cellulose bonds" and to fragment some cellulose molecules. The hydrolyzation then converted a portion of the cellulose made accessible in oxidation to saccharides and saccharide acids. He notes the oxidation step prior to hydrolyzation substantially increased sacchrification. Jelks' work is an extension of the earlier hydrolysis efforts criticized by Algeo. Rather than simply increasing accessibility, these workers carried out a full hydrolysis to sugar. Jelks' major contribution was in illustrating the beneficial effects of metal catalyzed oxidation in aiding hydrolysis. Table 2 lists his conditions.
TABLE 2 ______________________________________ Temperature Pressure Time ______________________________________ Oxidation 105-110.degree. C. 150 psi 15-20 mins. Hydrolyzation 180.degree. C. 135-150 psi 3-7 mins. ______________________________________
Bender (U.S. Pat. No. 4,136,207) described a low pressure, long residence time steam cooking process using a continuous plug-feed, plug-release reactor. He cited the economic benefits of lower pressure, which allow lighter equipment, but nevertheless noted that steam cooking can be applied through the full range of pressures. He also found that the use of chemical reagents was unnecessary. Table 3 lists his preferred times for aspen wood, a material very similar to straw, again based upon an understanding of his broad claim 8 that the shorter times are to be associated with the higher pressures.
TABLE 3 ______________________________________ Pressure Time ______________________________________ 210 1200 sec. 250 psig 300 sec. ______________________________________
Bender cautioned that cooking longer than this could lead to overcooking and consequent reductions in yield. It is interesting to note Bender teaches that oxidation, which Jelks found to aid hydrolysis, will actually decrease the yield of digestive material, thereby illustrating the basic disagreements in the prior art of this general technology.
More recently, Delong (British Application No. 2941.7/78, filed July 11, 1978 and published Jan. 17, 1979 as G.B. No. 2,000,822A) has proposed a Mason-type steam explosion process for fracturing the linkages between lignin, hemicellulose, and cellulose for the exact preferred food materials addressed herein, aspen wood chips, allegedly to make the cellulose and hemicellulose more readily accessible to both rumen micro-flora and enzymes. His material has the appearance of "potting soil" and "sinks like a stone in water".
Delong proposed a largely physical explanation for improved accessibility, i.e., since cellulose softens at a temperature of 230.degree. C., when wood is exploded in such a softened state, the fibre structure will simply be destroyed, thereby opening the cellulose and hemicellulose to attack.
Delong found that cooking at or above this 230.degree. C. temperature would only serve to promote an undesirable hydrolysis of the hemicellulose. Delong maintained that Algeo had substantially overcooked his material, causing a "drastic reduction in the fermentation value of the hemicellulose". Delong's stated objective was to make the cellulose highly accessible while minimizing this hemicellulose hydrolysis, and allegedly this was to be done by raising the temperature as rapidly as possible to the critical softening point, then immediately exploding the material.
Delong experimentally inferred that the best yields were obtained when the mass temperature (as measured by an unspecified probe projecting into the mass of chips) reached 238.degree. C. This was accomplished by adding 650 psi steam (258.4.degree. C. saturation temperature) to the reactor. In contrast, Bender noted (and this inventor's work can confirm), that such probe temperatures actually reflect only the saturated steam temperature in the reactor, and in Bender's continuous process no thermal probe response time characteristics were present. Thus, whereas previous workers had proposed steady-state cooking, Delong suggested exploding before reaching steady-state. The present invention clearly avoids such thermal measurement inaccuracies by adopting a more straightforward technique, sensing only reactor saturated steam pressure.
Algeo and Bender share a common goal with the present inventor, and certain similarities in the present inventor's basic approach, i.e., both Algeo and Bender seek to render the cellulose component of wood more accessible so that the output material can be used for future treatment, such as by enzymes or rumen bacteria. Hence, Algeo and Bender are relevant prior art, in that they are concerned with the treatment of natural wood, to increase the accessibility of the cellulose. While Delong also professed this same goal, Delong categorically took an approach which was to avoid any steady-state temperature circumstances, since Delong specifically sought to avoid any thermally-induced hydrolysis. (See Delong at page 3, lines 10+). Delong, which is the most recent of these three pertinent references, essentially taught away from inducing a thermal hydrolysis, and certainly did not teach how a specific reaction could be ensured by a cooking time that is a repeatable function of reactor pressure. Hence, the most relevant prior art appears to be the earlier teachings of Algeo and Bender, even though it is only the present invention which critically recognizes how a steady-state, thermally-induced partial hydrolysis mechanism can result in the optimization of cellulose accessibility, as most conveniently measured by the yield of glucose when the material is treated with a commercial cellulase enzyme under carefully controlled conditions. In order to prove the present invention, there follows various examples and graphic representations of how glucose yield increases to a surprising optimum, and how a sharp parameterization of the discovered mechanism was derived, to explain the surprising results.
Unlike the mutual goals of Delong, Bender, Algeo and the present invention, certain prior workers were not seeking to increase cellulose accessibility, but, rather, the distinct and separate goal of directly breaking down hemicellulose to produce xylose, and cellulose to produce glucose. In other words, the present invention categorically focuses on a "pre-treatment goal" wherein a range of reaction parameters are identified so that the cellulose becomes most accessible for any form of subsequent treatment. Exemplary subsequent treatments include using enzymes to break down the pretreated cellulose into glucose, or simply using the pretreated material in a direct manner as feed for animals, wherein the subsequent breakdown occurs in vivo, by the bacteria in the animal's stomach.
Hess et al. (U.S. Pat. No. 3,212,932) is typical of certain non-relevant prior art teachings which seek to produce glucose directly, and through the mechanism of using a relatively high concentration of mineral acid, to brutally break down all elements of the wood feedstock. By contrast (and as also noted by Delong at page 2, lines 16-22), the present invention avoids such harsh, acid hydrolysis, since the present invention teaches that it is far preferable to hydrolyze the relatively accessible hemicellulose only, and to a point where the hemicellulose degradation products do not, in turn, adversely affect the accessibility of the cellulose. Hess et al. employ a two-stage acid hydrolysis, the first stage to recover hemicellulose-derived xylose, the second to recover cellulose-derived glucose. In the first stage, finely-divided wood in the form of sawdust or wood shavings is mixed in a ratio of 1:1 to 1:3 with a treating liquor having sulphuric acid concentration of less than 0.3% and preferably zero. This "soup" mixture is then cooked, preferably at the conditions given in Table 4.
TABLE 4 ______________________________________ Pressure Time ______________________________________ 250 psig 600 sec. 600 psig 18 sec. ______________________________________
When this first cooking step of Hess et al. was complete, the pressure was rapidly reduced through a "flash blowdown". This, Hess et al. allege, served to stop the hydrolytic reaction and minimize the production of lignin degradation products, and to flash off acetic acid and other organic volatiles formed in the reaction. Delong confirmed this effect for the explosion from his Mason-type reactor.
By contrast, the present invention is, firstly, not performed in a "soup", but as dry chips surrounded by a steam envelope; and, secondly, Hess' pressure time parameters are not in theeenvelope shown for the present invention at FIGS. 2, 3.
Hess et al. then taught further treating the residue (from Table 4) with a 0.5% solution of sulphuric acid to remove the sugars produced. The solids are then mixed with a second treating liquor having a 0.3-3% acid concentration and cooked under the more severe conditions listed in Table 5.
TABLE 5 ______________________________________ Pressure Time ______________________________________ 400 psig 600 sec. 800 psig 18 sec. ______________________________________
The second stage hydrolysis serves to convert the remaining cellulose to glucose.
Thus, while the present invention shares with Algeo, Bender, and Delong the ultimate goal of increasing the accessibility of cellulose to rumen bacteria (or enzymes), Hess et al. had the entirely different goal of maximizing the hydrolysis yield of glucose and xylose. FIG. 1 shows the preferred cooking times of Algeo and Bender. Delong, on the other hand, believed that accessibility was a physical result of rapid decompression. Delong proposed transient heating followed immediately by steam explosion, so as to minimize hydrolysis, and as such would correspond to a "0" steady-state cooking time, is also shown on FIG. 1.